Canon-Puncture occurs when one character is told they are fictional by another character, for example: you meet Sherlock Holmes in 2012 and let him know that he's actually a fictional character (and even show him the body of work by Arthur Conan Doyle). It's considered bad form to do this.
The link lists seven steps if you recognize a fictional character (and conversely, are recognized as a fictional character), ranging from Don't Panic (applicable to both situations) to Don't Treat Him or Yourself Act Differently (applicable for most situations anyways).
With the number of characters available to most GMs, throwing in known fictional characters is always a fun bit, especially when the players don't expect it.
Gamma Red Death World
3 years ago